1. Field of the Invention
This invention relates to apparatus for driving a radiation detecting device for medical diagnostic purposes; and, especially, to apparatus for driving a scintillation camera to provide emission computerized tomography (ECT) capability for noninvasive medical diagnoses of a patient.
2. Description of the Prior Art
Emission computerized tomography is a noninvasive medical diagnostic procedure in which a head of a radiation detector precesses in an arc about a patient (for example, about the cranial-caudal axis of the patient), facing the part of the patient under study (e.g. a body organ) at all times. A commonly used radiation detector for ECT applications is an Anger-type scintillation camera (named for its inventor), the basic principles of operation of which are disclosed in U.S. Pat. Nos. 3,011,057; 3,732,419 and 3,984,689. The radiation detector computes the distribution of a radiation emitting substance previously ingested by the patient as detected from a plurality of viewing positions of the detector head, and analyzes this data to produce a profile of the object of study. In a typical ECT system, a single precession of the detector head about the patient produces a display showing the radioactive distribution in the object of study in a number of parallel section imaging planes.
Typical prior art structure is disclosed in U.S. Pat. No. 4,216,381. In that patent, a conventional, counterbalanced Anger-type scintillation camera is adapted for ECT analysis by structure which enables the camera head to be rotated in a circular orbit about a patient. The structure includes a base supporting an upright circular frame. The circular frame comprises an inner circular ring adapted for rotation within a concentric outer circular ring. Two parallel arms, tiltably attached at diametrically opposite balance points to the inner ring, carry the camera head at their one ends and a counterweight at their other ends. A drive system rotates the inner ring within the outer ring so that the points of attachment of the two arms to the frame are driven along a circular path whereby the detector head orbits the patient to receive emission data. The data is digitized and processed in electronic form. Using an appropriate algorithm, the computed radiographic distribution is constructed and displayed on a visual image display device. The radius of the arc which the head traverses is determined by the angle of tilt of the parallel arms at their points of attachment with respect to the inner ring. The angle of tilt is manually set and then locked prior to rotation by means of an electromagnetic brake. The manual setting of the tilt angle is aided by a pneumatic assist device connected between the inner ring and the arms adjacent the counterweight.
The prior art structure described in U.S. Pat. No. 4,216,381 is cumbersome and clumsy. Because the path of the head is controlled by parallel arms traveling about a ring, the apparatus is not compact and cannot be conveniently encased to give a pleasing aesthetic appearance which is important for patient ease in a hospital or doctor's office environment. Further, the points of attachment of the two arms that carry the head and the counterweight and also the brake that locks the tilt angle of the arms all rotate in a wide circle with the ring (the entire counterweight rotating about itself), thereby presenting increased risk of mechanical failure which may cause slipping and attendant inaccuracies in the data acquisition process or pose a safety hazard for the patient or the medical personnel in attendance.